Classful IP Addressing

Class	Starting Bits (fixed to m bits)	Decimal Range of First Byte	Network Bits n bits	Host Bits p bits	Max Networks 2^n-m	Max Hosts 2^p-2
A	0 (m-1)	1 to 126 (127 is loopback)	8	24	2^{8-1 =}126	2²⁴-2= 16,777,214
B	10 (m=2)	128 to 191	16	16	2^16-2 = 16,384	2¹⁶-2=65,534
C	110 (m=3)	192 to 223	24	8	2^24-3 = 2,097,152	2⁸-2=254
D	1110	224 to 239
E	1111	240 to 254

*** Class A, B, and C are for standard Internet IP addressing. Class D is for Multicast, and Class E are reserved for future use.

There are a total of 2³² = 4,294,967,296 possible addresses. Classful addressing, no longer used in the backbone but still referred to constantly, divided the total address space up into five Classes of addresses (Class A,B,C,D,E). The total number of unique addresses per Class depends on the number of bits used for the node (host) address.

Class A uses half of them (2,147,483,648 addresses)
Class B uses one-fourth (1,073,741,824 addresses)
Class C uses one-eighth (536,870,912)
Class D and Class E each use 1/16th - they split the rest of the addresses (268,435,456 each)

IP Address Components:

Like other network layer protocols, the IP addressing scheme is integral to the process of routing IP data through an internetwork. Each host on a TCP/IP network is assigned a unique 32-bit logical address. The IP address is divided into two main parts; the Network Number and the Host Number.

The network number identifies the network and must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part of the Internet. The host number identifies a host in the network and is assigned by the local network administrator.

IP Address Format:

The 32-bit IP address is grouped 8 bits at a time, each group of 8 bits is an octet. Each of the four octets are separated by a dot, and represented in decimal format, this is known as dotted decimal notation. Each bit in an octet has a binary weight (128, 64, 32, 16, 8, 4, 2, 1). The minimum value for an octet is 0 (all bits set to 0), and the maximum value for an octet is 255 (all bits set to 1).

The following figure shows the basic format of a typical IP address:

IP Address Classes:

IP addressing supports three different commercial address classes; Class A, Class B, and Class C. The following figure summarizes the network and host portion of each address class:

In a class A address, the first octet is the network portion, so the class A address of, 10.1.25.1, has a major network address of 10. Octets 2, 3, and 4 (the next 24 bits) are for the hosts. Class A addresses are used for networks that have more than 65,536 hosts (actually, up to 16,581,375 hosts!).

In a class B address, the first two octets are the network portion, so the class B address of, 172.16.122.204, has a major network address of 172.16. Octets 3 and 4 (the next 16 bits) are for the hosts. Class B addresses are used for networks that have between 256 and 65,536 hosts.

In a class C address, the first three octets are the network portion. The class C address of, 193.18.9.45, has a major network address of 193.18.9. Octet 4 (the last 8 bits) is for hosts. Class C addresses are used for networks with less than 254 hosts.

Determining the Class from the First-Octet:

The class of address can be easily determined by examining the first octet of the address, and mapping that value to a class range in the table below:

The left-most (high-order) bits in the first octet indicate the network class. For example, given an IP address of 172.31.1.2, the first octet is 172. 172 falls between 128 and 191, so 172.31.1.2 is a Class B address.

Classful Network Masks:

Each of the commercial address classes has a set classful network mask. The network mask defines which bits out of the 32 bit of the address are defined as the network portion and which are the host portion. The network mask is calculated by setting all bits to a value of 1 in the octets designated for the network portion and all bits to a value of 0 in the octets designated for the host portion.

As stated above, a Class A address has the first octet as the network portion and the remaining 3 octets as the host portion. Therefore, a Class A network mask is defined as 255.0.0.0.

A Class B address has the first and second octets as the network portion and the third and fourth octets as the host portion. A Class B network mask is shown as 255.255.0.0.

A Class C address has the first, second, and third octet as the network portion and the last octet as the host portion. A Class C network mask is shown as 255.255.255.0.